1. Field of the Invention
This invention relates to a method for the production of a salt-resistant absorbent resin. More particularly, it relates to a method for the production of an absorbent resin possessing high gel strength and improved salt-resistance.
2. Description of the Prior Art
The absorbent resins have been heretofore utilized in various absorbent materials represented by disposable diapers, sanitary articles, water-retaining agents for soil, and sealing materials.
These absorbent resins are broadly divided into those of the type possessing such an electrolytic structure as a carboxyl group and those of the type possessing a nonionic hydrophilic segment. As the absorbent resins of the former type, cross-linked acrylic acid (salt) polymer [JP-A-55-84,304(1980)], hydrolyzed starch-acrylonitrile graft copolymer (U.S. Pat. No. 3,661,815), neutralized starch-acrylic acid graft copolymer (U.S. Pat. No. 4,076,663), and saponified acrylic ester-vinyl acetate copolymer (U.S. Pat. No. 4,102,842), for example, have been known. As the absorbent resins of the latter type, modified cross-linked polyvinyl alcohol [JP-A-54-20,093(1979)] and partially cross-linked polyethylene oxide [JP-A-61-1309324(1986)], for example, have been known.
The absorbent resins by nature have found utility in a wide range of industrial fields in need of them for the purpose of absorbing various aqueous solutions. But their absorbency varies to a large extent by the kinds of liquids to be absorbed. When they are used in disposable diapers, for example, their absorbency are varied by changes in salt concentration of urine and diaper qualities are consequently also varied. Their absorbency is affected by the salt concentration, so there are times when these absorbent resins find restricted utility in the field of agriculture and horticulture or in the application to water-repellent agents.
The absorbent resins of the type possessing an electrolytic structure such as a carboxyl group, for example, generally possess high gel strength and exhibit a very high absorbency as to deionized water. They, however, have the problem of poor resistance to salts as evinced by their notably low absorbency exhibited to electrolytic solutions such as an aqueous common salt solution.
The absorbent resins of the latter type possessing a nonionic hydrophilic segment are excellent in terms of salt-resistance as evinced by small decreases in their absorbency exhibited to electrolytic solutions. They, however, possess weak gel strength, betray low speeds of absorption, and show only low absolute values of absorbency.
As an excellent salt-resistant resin the absorbent resin incorporating therein a sulfonic acid group which is a strong electrolyte [JP-A-61-36,309(1986) and JP-A-56-161,412(1981)] has been known. Though this resin manifests an outstanding resistance to liquids containing polyvalent ions, it is as deficient in salt-resistant ability in liquids containing monovalent ions. Moreover, this resin is expensive because it uses as a raw material a monomer possessing a sulfonic acid group.
An object of this invention, therefore, is to provide a method for the production of a salt-resistant absorbent resin.
Another object of this invention is to provide a method for inexpensive production of an absorbent resin possessing high gel strength and yielding very slightly to variation in absorbency due to the salt concentration in an aqueous solution subjected to absorption.